Liquid chillers and direct expansion air conditioners constitute virtually all cooling systems for buildings beyond natural ventilation and evaporative cooling systems. Chillers and direct expansion air conditioners operate by absorbing heat from the space being cooled either directly (direct expansion air conditioners) or by circulating a secondary fluid (chillers). Rejecting the heat that has been absorbed and has been generated by the cooling apparatus is almost universally accomplished by transferring the heat to the environment outside the building or space. FIG. 1 illustrates the major components of typical compression cycle cooling apparatus. In this system, a motor (109), drives the compressor (110), which draws low pressure refrigerant gas from the cooler (124) through a suction line (130), compresses it, and discharges it as a higher pressure hot gas through a hot gas line (112) into the condenser (114). In the condenser, the hot gaseous refrigerant is condensed into a liquid by rejecting heat to outside air by blowing outside air across the condenser with a fan (116) driven by an electric motor (134). The condensed liquid refrigerant flows through an expansion device (122) that regulates the flow of refrigerant into the cooler also called the evaporator (124), which is held at a low pressure by the operation of the compressor. The low pressure environment causes the refrigerant to change state to a gas and as it does so, it absorbs the required heat of vaporization from the chilled water or air circulating through the cooler, entering at (135) and leaving at (136). The low pressure refrigerant vapor is drawn into the inlet of the compressor and the cycle is continuously repeated. Usually such cooling apparatus has some method of regulating cooling capacity for part load operation such as a modulating scroll or vane apparatus (111) which limits the amount of refrigerant through the compression device, or a variable speed apparatus (108) which controls the rotational speed of the compression device, or both. The chilled water or chilled air is circulated through a distribution system for comfort conditioning, or to provide cooling for certain processes within the building. In this circuit, the heat absorbed from the cooler along with the heat added by the compressor is rejected to the outside air.
FIG. 2 illustrates a compression type cooling apparatus of the same type as FIG. 1 except that it employs an evaporative cooling device to reject heat into the outside air. In this FIG., hot gaseous refrigerant enters the condenser (209) and heat is rejected such that the refrigerant condenses to a liquid. In this condenser, tepid water from the sump (211) of a cooling tower (210) is circulated through the condenser by a pump (212), and then to spray nozzles or a distribution flume (213) which distributes the water over slats or plastic fill (214) that breaks the water up into droplets with a very large surface area such that a fan (215) driven by an electric motor (216) forces air over the water, evaporating a portion of it to provide adiabatic cooling of the water. The cooler water gathers in the sump where the water lost through evaporation is made up by adding water from a water supply (217). The level of water in the tower sump is maintained by a water level sensor (218), which operates a valve (219). Water in the sump is drawn through the condenser to provide continuous rejection of the heat absorbed from the space and that generated by the cooling apparatus which functions as described in FIG. 1. In addition to compression type chillers and air conditioners that absorb heat at low temperature and rejects it at higher temperatures is the absorption type chillers and air conditions whose cycle operates differently, but employ coolers and condensers to absorb and reject heat in the same fashion as the compression type described.
It is estimated by the Electric Power Research Institute that approximately 900 billion kilowatt hours are used annually by commercial buildings in the U.S. and about 3% to 5% of that energy is expended to reject heat from air conditioning systems for commercial buildings alone.
The present state of the art employs several methods for controlling the condenser fan in the direct air-cooled systems as illustrated in FIG. 1. In many of the smaller air conditioners (such as window units), that are employed to cool rooms, single offices, or houses, the condenser fan operates continuously at full speed whenever the cooling apparatus is operating. It is also established to operate multiple condenser fans based on refrigerant pressure (U.S. Pat. No. 5,138,844), and the use of variable speed condenser fans is known to adjust flow for non-azeotropic condensers (U.S. Pat. No. 5,385,030) in response to temperature changes.
Similar temperature and pressure control strategies are known and employed to control the condensing circuits for evaporative cooled condensers (FIG. 2). This type of heat rejection is almost entirely employed in larger systems that are employed to cool commercial facilities. In these systems there are two energy consuming components in the heat rejection circuit, the condenser pump which circulates water through the condenser and cooling tower, and the tower fan which forces air over the water in the tower and provides adiabatic cooling of the water by evaporating the water into that air stream.
The present state of the art for evaporative type condenser cooling circuits involve special control for the tower fan only. It is known how to sequence two speed fans to maintain a tower water temperature setpoint (U.S. Pat. No. 4,085,594) and to control the speed of the fan to maintain the tower water temperature setpoint (U.S. Pat. No. 4,252,751 and U.S. Pat. No. 4,554,964). However, in order to assure stable chiller operation, it has been (and continues to be) strongly recommended by manufacturers of chillers that the condenser water flow be maintained at a constant rate at all chiller load conditions. Optimization of heat rejection has been limited to regimens for controlling only the tower fans. For example, U.S. Pat. No. 5,600,960 which calculates a near optimal tower leaving water temperature and operates the tower fan to maintain that temperature, and U.S. Pat. No. 5,040,377 which calculates an optimal total air flow, and operates single and multiple cooling tower fans to maintain that percent of maximum air flow. Also U.S. Pat. No. 4,474,027 employs a tower water setpoint based on the current outdoor wet bulb temperature. In all these control schemes, it is assumed that the condenser water pumps operate continuously at a single (full) speed and flow. However, I have observed that condenser pump(s) typically consume more power than the tower fan.